The present invention generally concerns the television art and the processing of synchronizing signals within the television receiver. More particularly, the present invention concerns the separation of such signals from the transmitted video signal under varying signal conditions.
The video picture image transmitted by a television video signal is reproduced on a television screen through the use of synchronizing signals which are transmitted on the same carrier signal and serve to regulate the horizontal and vertical sweeping action of the picture tube. This action allows the video image to be traced in a raster-like fashion on the viewing screen. The frequency and timing standards for these signals are set by the N.T.S.C. to achieve industry-wide uniformity and compatibility between tranmsitting stations and the different brands of home television sets.
In practice, a composite sync signal bearing horizontal and vertical sync information is superimposed on a video carrier signal. In a standard signal this results in regularly spaced sync pulses having video information imposed between the synchronizing pulses. There is no video information during the pulses themselves; horizontal and vertical sync pulses occur during horizontal and vertical blanking periods, respectively, in which the picture tube scanning beam is returned to the left side and upper left corner, respectively. Also, the sync pulses typically have a higher D.C. component than the video information. Thus, for a television receiver to be effective, it needs to separate the sync information from the video information which can be done in a number of ways relying on the known sync signal characteristics.
This operation has long been performed in the television art and frequently has been accomplished using a conventional sync filter as shown in FIG. 1. This is a pure analog filter which requires a capacitor 10, two resistors 12 and 14, a diode 16, and an electrolytic capacitor 18. Although effective, this is not the most ideal method for separating the sync information from the remainder of the video signal. Particularly, problems can arise in television reception when this traditional configuration is used if a weak or noisy signal is received; the sync tips of the synchronizing pulses may move toward the porch and video levels of the carrier signal thus making them harder to distinguish.
Also, problems can arise when a television receiver is used in conjunction with a home video game, video cassette recorder, or computer, frequently described in the art as non-standard operation. The outputs of video games and computers have synchronizing signals which frequently differ significantly from those specified in N.T.S.C. promulgated standards. Often these pulses are longer or shorter in duration than standard horizontal synchronizing pulses thus creating difficulties for other circuits in the television receiver, particularly the Horizontal A.P.C. circuit. Likewise, the vertical sync pulses present during the serrated vertical period of the video signal may be much longer or shorter in duration than horizontal sync pulses or standard vertical sync pulses and may also cause difficulties in other circuits.
Thus, it is desired that a sync separator or filter be able to accurately separate or pick out the composite sync signal from among the rest of the video signal even during noisy and weak signal conditions when the sync information is less clearly defined in contrast to the remainder of the video signal. Also, it is desired that a sync separator not fail to recognize sync information when it is received in nonstandard form. Additionally, it is desired that at the sync separation stage, some provision be made to protect ensuing circuits against longer enduring sync pulses and thereby ensure the accuracy of those circuits. Finally, it is desired then to overcome these problems while at the same time minimizing cost, by eliminating external parts.
Therefore, it is an object of the present invention to provide for a digital sync separator utilizing a minimum of external components.
It is a further object of the present invention to provide an improved digital sync separator for operation during noisy, weak, and nonstandard video signal conditions without sacrificing performance during standard operation.
It is still a further object of the present invention to provide a sync differentiator for imposing at least a minimum of standardization in the composite sync signal within the receiver for other circuits such as the Horizontal A.P.C.
It is a related object of the present invention to provide conditioning of the composite sync signal for further use in the television receiver.
It will be appreciated that a still further object of the present invention is to provide an improved digital sync separator and differentiator with a minimum of cost.